Flame ionization detector



April 14, 1964 L. ONGKIEHONG ETAL FLAME IONIZATION DETECTOR Filed May 2,1961 2 Sheets-Sheet 1 ll l2 FIG. 5

INVENTORS:

LEO ONGKIEHONG AART BIJL ALBERTUS SCHURINGA BY f/W THEIR ATTORNEY April1964 L-. ONGKlEHONG ETAL 3,129,052

FLAME IONIZATION DETECTOR Filed May 2. 1961 I 2 Sheets-Sheet 2 wal FIG.4C

AART BIJL ALB ERTUS SCHURINGA BYQWZ/JW THEIR ATTORNEY United StatesPatent FLAME IONIZATION DETECTOR Leo Ongldehong, Aart Bijl, and AlbertusSchuringa, all of Amsterdam, Netherlands, assignors to Shell GilCompany, New York, N.Y., a corporation of Delaware Filed May 2, 1961,Ser. No. 107,165 Claims priority, application Great Britain May 6, 19607 Claims. (Cl. 23--255) The invention relates to a circuit including aflame ionization detector for the examination or analysis of gases,particularly gases obtained by a chromatographic separation.

It is known to investigate or analyze gases by means of a flameionization detector, particularly to use a detector of this kind in thetechnique of chromatographic separation.

In a flame ionization detector use is made of a burning gas, usuallypure hydrogen or a mixture of gases of which hydrogen is an integralpart, in the form of a small flame whose electrical conductivity ismeasured by means of two electrodes disposed in the plasma of the flame.The gas to be examined or analyzed is supplied to the flame.

Of the electrodes inserted in the plasma of the flame one is usuallyformed by the burner (which in this case is made of metal) and the otherby a gauze or wire of platinum or bronze positioned about 10 mm. abovethe burner. In principle, however, the electrodes may also be arrangedtransversely to the flame. The flame itself is usually not greater thana few millimeters.

The conductivity of the hydrogen flame is very low on the order ofmagnitude of 10 to 10* mhos. Even the addition of small amounts oforganic material is, however, suflicient to greatly increase theconductivity of the flame; depending on the concentration of organiccomponents in the flame the conductivity may be 10 to as great. Thechanges in the conductivity can be measured and if necessary recorded;the result of the measurement is an index of the presence orconcentration of organic material in the gases supplied to the flame.

If the flame ionization detector is used as a detector behind a gaschromatography column in which hydrogen is used as the carrier gas, thegas from the column can be directly burnt in the detector; if anothergas such as nitrogen is used as the carrier gas the gas in the column isfirst mixed with hydrogen and then supplied to the detector.

The conductivity is usually measured by connecting a direct voltagesource and a resistance in series with the electrodes and measuring thevoltage over the measuring resistance. Since the absolute value of theresistance of the flame is very high the measuring resistance will alsohave to have a high value in order to obtain measurable signals. Themeasuring resistance should, however, not be too high compared to theflame resistance to ensure that the magnitude of the current in theseries connection is determined by the detector; in this case variationsin the flame resistance give substantially proportionate variations ofvoltage over the measuring resistance.

The signal over the measuring resistance may also be recorded and/orindicated and thus is usually amplified to provide suificient signal todrive a chart recorder. Owing to the high impedance level at which thesignal becomes available it is, however, impossible to supply the signaldirectly to a recording or indicating device. The signal is thereforefirst amplified by means of a direct voltage amplifier. To obviate asfar as possible the trouble caused by the drift to which all directvoltage amplifiers are subject, the input signal of the direct voltageamplifier, notwithstanding the amplification employed, should, however,be as large as possible, so that the measuring resistance should havethe highest possible value. In practice, these difliculties have beensolved by using an electrometer amplifier in combination with ameasuring resistance of 10 to 10 ohms, depending on the concentrationrange of the gases to be examined in the flame of the detector.

Electrometer amplifiers are, however, expensive, fragile and costly tomaintain. Moreover, there is the drawback that their response velocityis small, thus the reproducibility of relatively rapid variationsoccurring in the gas being examined is diflicult.

Accordingly, the principle object of this invention is to provide anovel detecting system for a gas chromatograph unit using an alternatingvoltage amplifier.

A further object of this invention is to provide a unique flameionization detecting unit for a gas chromatograph that permits the useof an alternating voltage amplifier.

A further object of this invention is to provide a novel flameionization detecting unit having a third or control electrode, with thecontrol electrode being coupled to an alternating voltage source.

A still further object of this invention is to provide a flameionization detecting unit utilizing a control electrode coupled to analternating voltage source with the signal from the detecting unit beingsupplied to a rectifying circuit that passes only the positive peaks ofthe signal.

This invention provides a circuit using a flame ionization detector bymeans of which a simple, low cost, alternating voltage amplifier may beused. The circuit also shows a much faster response and, consequently,detects rapid variations in the composition of the gas to be examined oranalyzed, this being particularly advantageous when the flame ionizationdetector is used as a detector in chromatography technique and in gaschromatography in particular.

According to the invention, a third electrode (control electrode) isplaced in the iield between the electrodes of the detector. The controlelectrode is coupled to a varying voltage, preferably a periodicallyvarying voltage that temporarily interrupts the current between the twoelectrodes, or at least partly suppresses the said current. An outputsignal is derived from the output of the detector, preferably via analternating voltage amplifier. The magnitude of this output signalconstitutes the result of the measurement and may be recorded and/ orindicated either directly or after rectification.

The control electrode need not, as regards its spatial arrangement, bepositioned exactly between the two other electrodes, since it issufiicient to place the control electrode in such a way that itspotential influences the current between the other electrodes in themanner indicated. In practice, the control electrode temporarilycompletely suppresses the current of one of the other electrodes.

In addition, an auxiliary electrode for screening is preferably arrangedin the field between the control electrode and one of the otherelectrodes, the said auxiliary electrode being incorporated in thecircuit in such a way that direct capacitive currents from the controlelectrode to the detector output are reduced or eliminated.

The above objects and advantages of this invention will be more easilyunderstood from the following detailed description of specificembodiments when taken in conjunction with the attached drawings inwhich:

FIGURE 1 is a block diagram of one embodiment of this invention using aflame ionization detector having only a control electrode;

FIGURE 2 is a block diagram of a second embodiment of this inventionshowing the use of two detectors having both control and screenelectrodes in order that the difference between the two detector signalsmay be measured and recorded;

FIGURE 3 shows the detailed construction of a flame ionization detectorin accordance with this invention;

FIGURE 4 shows the detailed construction of the electrodes of thedetector shown in FIGURE 3; and,

FIGURE 5 is a schematic diagram showing a third embodiment of thisinvention.

The flame ionization detector system is shown diagrammatically in FIGURE1 in which the detector consists of a housing 1, burner 2, and an airsupply aperture for the flame 3. The housing 1 is electrically connectedto ground while the mixture of hydrogen gas and the gas to be examinedis supplied at 4 with the combustion air being supplied at 5. The metalburner acts as one electrode (burner electrode), while above the burneris placed a platinum gauze 6 forming the other electrode (collectorelectrode) that is electrically insulated from the housing 1. Thecollector electrode is connected to earth via a source of direct voltage7 and a measuring resistance 8.

According to the invention a third electrode 9 is also arranged in thedetector in the field between the burner electrode 2 and the collectorelectrode 6, with the third electrode, being electrically insulated fromthe housing 1. The electrode 9 is connected to the burner electrode 2via a source 10 which supplies a varying electric voltage. The purposeof this voltage is the repeated temporary suppression (complete orpartial) of the ionization current in the circuit 2-67S. The magnitudeof the voltage 10 is preferably such that the current is completelyinterrupted. The voltage 10 need not be a sinusoidal a1- tern-atingvoltage 'but may, for example, be of a pulsating voltage of arectangular nature. A direct voltage may also be coupled to theelectrode 9.

Whenever the amplitude of the voltage 10 exceeds a certain value theionization current in the collector circuit is interrupted. This occursboth when the voltage of electrode 9 is positive and when it is negativewith respect to the electrode 2. The value of the positive voltagerequired to suppress the current in the collector circuit may vary fromthe value of the negative voltage required to suppress the current. Toavoid the asymmetry in the interruption a rectified, a non-filteredalternating voltage may he used for the source 10 of varying voltage.

The result of the interruptions is that an alternating voltage isestablished across the resistance 8 which is a measure of theconductivity of the flame. This alternating voltage can be amplified ina simple manner by an alternating voltage amplifier 1 1 and indicatedand/or recorded by an indicator and/or recording instrument 12.

The great advantage of the present circuit is the possibility which itaffords of employing an alternating voltage amplifier instead of adirect voltage amplifier. Such an amplifier is much cheaper and iscapable of stable operation at a much lower input voltage than a directvoltage amplifier. Consequently, the value of the measuring resistance 8may be relatively low (about 100 times smaller than with thecorresponding use of the known circuit with a very good direct voltageamplifier), as a result a much more rapid response (100 times morerapid) of the instrument is obtained and hence it is possible toindicate more rapid variations in the conductivity of the flame.

A second embodiment of this invention is shown in FIGURE 2 which,however, differs in two respects from the embodiment of FIGURE 1. Thecircuit is of a double design: the detector 1 has the same function asthe detector 1 in FIGURE 1, and the other detector 1 is fed with thehydrogen gas without the gas to be examined. The difference in the twoionization currents sets up a current over the measuring resistance 8.To this end the direct voltage sources 7 and 7 are connected in thopposite directions to the detectors 1 and 1'.

In addition, extra screening electrodes 13 and 13' are connected betweenthe electrodes 9 and 9' and electrodes 4 6 and 6'. The screeningelectrodes in turn are connected to the electrode 2 and 2. The use ofscreening electrodes 13 and 13' ensure that capacitive currents from thecontrol electrodes 9 and 9' to the detector output resistance 3 aresubstantially blocked, thus benefiting the accuracy and sensitivity ofthe measurement.

In order to give the measuring instrument a number of measuring rangesthe magnitude of the measuring resistance 8 may be modified by means ofa switch 14.

The direct voltage sources 7 and 7' are provided with a shield so thatleakage currents from the batteries 7 and 7 are not reflected over themeasuring resistance.

The above-mentioned circuits are particularly suitable for use as adetector circuit behind a column for gas chromatography. The gasesderived from the column may be supplied to the hydrogen streamintroduced through pipe 4; if hydrogen is already used as a carrier gasin the column, the gas stream from the column may be directly connectedto 4, in which case a special hydrogen stream is usually unnecessary.

In another use the detector circuit according to the invention may beemployed for measuring the concentration of hydrocarbons in theatmosphere. In this case the air to be examined may be mixedcontinuously or batchwise with the hydrogen stream introduced at 4. Inprinciple, it is also possible to supply this air at 5 in the form ofcombustion air, although this method is less sensitive.

The combustion air normally supplied at 5 is usually passed through afilter which prevents solid particles from entering the flame with pureair or pure oxygen preferably being used. The hydrogen supplied at 4 maybe pure hydrogen, although it may also contain an amount of other gasessuch as nitrogen, argon, helium or the like. Use is frequently made of amixture comprising 50% hydrogen and 50% nitrogen.

The frequency of the periodically varying voltage may be ordinary60-cycle-per-second voltage, but under certain circumstances it may alsobe lower or higher and even considerably higher. A frequency higher thanabout 1000 cycles per second is usually, however, less suitable owing tothe capacity which is present parallel to the measuring resistance 8.The direct voltage source 7 may have a value of, for example, volts,while the peak value of the auxiliary voltage 10 may be 50 to 60 volts,for example. The distance of the electrodes 2 to 6 is usually about 10mm.

It is, of course, not strictly necessary to connect the burnerelect-rode to ground and one of the other electrodes, for example thecollector electrode, may be grounded if it is desired.

The electrodes may also be arranged transversely to the flame. In thiscase electrodes are then preferably arranged concentrically around theflame in order that they envelope the flame. In this way a very compactconstruction may be obtained.

A construction of this type is shown in FIGURE 3, in which the burnerconsists of two concentric pipes 30 and 31. The combustible gas isgenerally passed through the inner pipe 31 and the combustion airthrough the annular space 32 between the two pipes, although thisprocedure may also be reversed. The two streams may even be previouslymixed and then burnt, although the effect is slightly less favorablewith this method.

The control electrode 33, the screening electrode 34 and the collectorelectrode 35 are cylindrical and arranged concentrically around theburner. They may be fabricated by various means, preferably etching from0.2 mm. thick stainless steel. FIGURES 4a, 4b and 4c show developedviews of these electrodes. The various electrodes are secured to a disc36 made of insulating material for example ceramic material. Inaddition, the system is surrounded by a shield (not shown) which ensuresthat it is electrostatically screened and also prevents false aircurrents from reaching the flame. The

system shown in FIGURE 3 has a diameter of approximate 45 mm., althoughit is quite possible to build a smaller system, for instance one-halfthis size.

The interruption of the ionization current by a voltage 10 having afrequency of 60 cycles per second sets up an alternating voltage of 120cycles per second over the measuring resistance which afteramplification constitutes the output signal. With the use of analternating voltage of 25 volts on the control electrode and a directvoltage of +400 volts on the collector electrode a sensitivity of 10*amp. is obtained for l p.p.m. of n-butane in a gas stream consisting of50 percent hydrogen and 50 percent nitrogen with the gas stream beingadjusted to a constant value of 1.5 cc./sec. The screening electrodeminimized cross-talk from the control electrode to the collectorelectrode caused by the second harmonic of the voltage 10 to a value of3.10 amperes.

It was, however, found that the above circuit could only be used if theconcentration of the gas to be examined or analyzed in the hydrogenflame was less than that corresponding to about 200 p.p.m. of n-butane.If this limit was exceeded the linearity of the output signal decreasedsince the sensitivity of the detector decreases with higherconcentrations of the gas in the hydrogen flame. This results from spacecharge effects between the electrodes. After an interruption by thepositive half of the voltage 10 the subsequent ionization current peakthrough the measuring resistance is in fact found to be a true measureof the concentration in the flame of the gas to be measured; but afteran interruption by the negative half of the voltage 10 the ionizationcurrent peak is not saturated, and depends on both this concentrationand on the space charges still present in the field at that moment. Inother Words, the first peak is a saturated current of which the peakvalue merely depends on the charge carriers produced by the flame, butthe second peak is not saturated on account of diffusion andrecombination effects and hence does not soley depend on the chargecarriers produced by the flame at that moment.

In order to obtain an output signal with optimum linear dependency onthe said concentration it is advisable to ensure that only thefirst-mentioned current peaks contribute to the output signal. Theoutput signal is preferably solely determined by the maximum values ofthese peaks (top detection). This may be achieved by couplingalternating current amplifier to a circuit which in the frequency of thevoltage 10 passes only the desired series of peaks and which blocks theother series. If, moreover, the passthrough phase is restricted to themoments at which the peaks have reached their maximum values, thedesired top detection is obtained. Smoothing of the peaks andsuppression of the frequency of the voltage 10 then results in an outputsignal which is, in fact a measure of the concentration of the gases inthe flame. In this way an output signal could be produced forconcentrations up to a 1000 p.p.m. of normal butane which is stillentirely linear, the sensitivity being 10- amp. for a concentration of 1p.p.m. of normal butane.

If the above-described method is employed it is highly desirable toensure the complete absence of cross-talk from the control electrode tothe collector electrode, since in the above case of top detection thecollector current peaks have the same frequency as the voltage 10. Hencea low voltage which may be a controllable voltage derived from thevoltage source 10 is preferably supplied to the screening electrode 13of FIGURE 2.

FIGURE shows a complete circuit as used when peak detection is employed.The circuit essentially corresponds to the one shown in FIGURE 1 withthe addition of a screen electrode 13.

The voltage source may be ordinary 120-volt 60- cycle power which iscoupled to the control electrode 9 by a transformer and in oppositephase to the screening electrode 13. The secondary winding of thetransformer includes a center tap which is grounded. The secondary ofthe transformer is coupled to the screening electrode via an adjustableresistance 16. Suitable adjustment of the magnitude of the compensatingvoltage by means of the resistance 16 substantially prevents cross-talkfrom the control electrode to the collector electrode.

A circuit 17 consisting of a bridge circuit formed of four transistors18 is coupled to the alternating current amplifier 11. The conduction ofthese transistors is determined by the voltage 19 between the base andcollector of each of these transistors. A pulsating voltage derived froma monostable multivibrator (not shown in the drawing) is connectedbetween the terminals 20 and 21 of a transformer 22. This voltage ispassed to the primary winding of a transformer 22 with two secondarywindings supplying the voltage 19. The frequency of the multivibratorvoltage is the same as that of the alternating voltage 10. The voltagepreferably consists of sharp rectangular pulses having a width ofapproximately 1 msec.

During the period of the pulse of voltage 19 the circuit 17 isconductive (resistance in the order of magnitude of 10 ohms) and duringthe remainder of the period the circuit 17 is not conductive (resistancein the order of magnitude of a few thousand ohms. The moment at which apulse arrives may be so selected as to coincide with the appearance ofthe top of the desired current peak. In this way the top of the curentpeak is allowed to pass through while other signals are blocked. Thepeaks are smoothed by means of a condenser 23 and the resultant varyingdirect current is then passed through a filter 24 cutting off thefundamental frequency. The filter is usually so selected that only thevariations in the direct current which are lower than 5 to 10 cycles persecond are transmitted.

A circuit, such as a mechanical or electronic interrupter controlled inthe desired manner could, of course, be used instead of the circuit 17.Moreover, the upper (or lower) half of the circuit 17 shown issufficient and in this case the resistance in the pass-through periodand the blocking resistance in the blocking period would be doubled.

The sensitivity of the equipment may be controlled, for example bycontrolling the measuring resistance 8. The full scale deflection of theindication may, for example, correspond to 20 p.p.m. of n-butane 1%noise), but may also be adjusted to a lower value (1 p.p.m. of n butane)or a much higher value. If a high sensitivity is desirable the flameionization detector will generally be given the smallest possibledimensions.

We claim as our invention:

1. A flame ionization type of detector system for analysis of gasesobtained by a chromatographic separation comprising: a detector having afirst electrode; a burner assembly, said burner assembly forming asecond electrode a control electrode disposed between said first andsecond electrode; a source of alternating voltage coupled to saidcontrol electrode and said second electrode; a source of direct voltagecoupled to said first and second electrodes and an alternating voltageamplifier coupled to said first and second electrodes to detect thesignal appearing across said first and second electrodes.

2. The system of claim 1 in which a screen electrode is disposed betweenthe control electrode and one of the spaced electrodes, said screenelectrode being coupled to the other of said spaced electrodes.

3. The system of claim 1 in which the output side of the amplifier iscoupled to a rectifying circuit that passes signals appearing on saidspaced electrodes as a result of the positive phases of said alternatingvoltage source and blocks all other signals.

4. The system of claim 3 in which the rectifying circuit includes agating circuit having the same frequency as said alternating voltage,said gating circuit being adjusted to pass only the peak value of thesignal.

5. The system of claim 3 in which the rectifying circuit consists of atransistor bridge circuit and a source of pulsating voltage having asquare wave form coupled to said transistor bridge to bias saidtransistors at the same frequency as the alternating voltage coupled tothe control electrode.

6. The system of claim 2 in which the detector consists of threecylindrical electrodes arranged concentrically around a burner, saidburner being arranged to burn the gas discharged from a chromatograph,said burner in addition forming the fourth electrode.

7. A flame ionization detector comprising: a central burner arranged toburn the gas discharged from a chromatograph unit said burner formingone electrode; a

cylindrical collector electrode disposed concentrically around theburner; a cylindrical control electrode disposed concentrically aroundsaid burner between the burner and the collector electrode; and acylindrical screen electrode disposed concentrically around said burnerbetween said collector and control electrodes.

References Cited in the file of this patent UNITED STATES PATENTSAndreatch et al Mar. 27, 1962 3,039,856 McWilliam June 19, 19623,049,409 Dower Aug. 14, 1962

1. A FLAME IONIZATION TYPE OF DETECTOR SYSTEM FOR ANALYSIS OF GASESOBTAINED BY A CHROMATOGRAPAHIC SEPARATION COMPRISING: A DETECTOR HAVINGA FIRST ELECTRODE; A BURNER ASSEMBLY, SAID BURNER ASSEMBLY FORMING ASECOND ELECTRODE A CONTROL ELECTRODE DISPOSED BETWEEN SAID FIRST ANDSECOND ELECTRODE; A SOURCE OF ALTERNATING VOLTAGE COUPLED TO SAIDCONTROL ELECTRODE AND SAID SECOND ELECTRODE; A SOURCE OF DIRECT VOLTAGECOUPLED TO SAID FIRST AND SECOND ELECTRODES AND AN ALTERNATING VOLTAGEAMPLIFIER COUPLED TO SAID FIRST AND SECOND ELECTRODES TO DETECT THESIGNAL APPEARING ACROSS SAID FIRST AND SECOND ELECTRODES.